1. Field of the Invention
The invention generally relates to hybrid transmission apparatuses, and particularly relates to a hybrid transmission apparatus which establishes synchronization between SONET apparatuses and SDH apparatuses.
2. Description of the Related Art
Transmission apparatus (SONET/SDH apparatus) that constitutes an SDH (synchronous digital hierarchy) network or an SONET (synchronous optical network) attends to the control of a network, such as ring switching at the time of channel failure, only at the SONET/SDH layer by monitoring, terminating, and inserting the overhead of an SONET/SDH frame.
In WDM (wavelength division multiplexer) networks, also, only the WDM layer is subjected to network control. In IP (internet protocol) networks, further, network control is carried out only between routers that switch IP packets, i.e., only at the TCP/IP layer.
Against this background, talks for the standardization of RPR (resilient packet ring) that has the function to switch rings have recently been started at IEEE (U.S. electrical and electronic engineer association) with an aim of achieving the integrated network control of an IP network and an optical network. Moreover, various apparatuses and designs have been proposed as a RPR peer model (e.g., Japanese Patent Application Publication No. 2001-160840).
FIG. 1 is a block diagram showing an example of a related-art transmission apparatus having the RPR function. In FIG. 1, receiving units 1W and 1E and transmitting units 2W and 2E are connected to optical transmission lines of a LAN. Signals received by the receiving units 1W and 1E are routed by a switching engine 4 by referring to topology information stored in CAMs (content addressable memories) 3W and 3E, respectively. These signals are then supplied to the transmitting units 2W and 2E through ring switching, and are transmitted to the LAN.
A considerable number of WDM apparatuses, ADM (add drop multiplexer) of the SONET/SDH, cross-connect apparatuses, ATM (asynchronous transfer mode) nodes, etc., are already in existence. Despite this, communication providers need to replace existing facilities when introducing routers or the like having the RPR function. This forces the communication providers to bear significant expenses. If integrated network control can be achieved by partial modification to existing apparatuses, merits are enormous. Such apparatus design, however, has not been in existence. Further, it is cost effective if SONET/SDH apparatuses, WDM apparatuses having no OXC (optical cross connect) function, and OADM (optical add drop multiplexer) apparatuses can be upgraded. Such apparatuses, however, have not yet been designed.
The related-art transmission apparatus shown in FIG. 1 attempts the integrated control of networks at the RPR layer by consolidating all optical network topology configurations such as SONET/SDH and WDM. To this end, topology information about all networks needs to be stored in the database of the CAMs 3W and 3E and to be exchanged with other transmission apparatuses, thereby integrating the CAM databases of all the transmission apparatuses. If the size of networks is large, the database of the CAMs 3W and 3E grows large. Further, the processing load on the switching engine 4 becomes heavy because of the need to coordinate operations with the other transmission apparatuses.
Moreover, if network control is performed by controlling RPR, the provision of the RPR function in all the transmission apparatuses in the optical network is required for the purpose of achieving the control of the entirety of the network. This entails the replacement of existing SONET/SDH/WDM facilities. Such a scheme thus offers little merits to communication providers.
Moreover, failures occurring at the WDM or SONET/SDH layer may be collected for analysis, and automatic switching may be made to cope with channel failures. Such processing, however, may fail to be finished within a satisfactorily short period such as 50 ms that is achievable by the existing SONET/SDH apparatus.
Accordingly, there is a need for a transmission apparatus which can perform ring switching at the SONET/SDH level and ring switching at the RPR level independently of each other.